Liquid crystal display devices, whether passive or active, customarily embody thin, parallel, spaced glass panels with an intermediate liquid crystal layer.
Initially, soda lime glass panels were used in producing passive LCD devices. It was observed that, when such panels were employed, degradation of the liquid crystal occurred at normal operating temperatures. This was due to sodium ion migration to the glass surface, and an exchange of sodium for hydrogen ions in water contaminating the liquid crystal. To avoid this, it has been proposed to apply a silica film to the surface of the glass panel. This film acts as a barrier layer to stop sodium ion migration from the glass, and thus prevent exposure of the liquid crystal layer to the sodium ions.
Another approach to controlling sodium has been to use a BaO--Al.sub.2 O.sub.3 --B.sub.2 O.sub.3 --SiO.sub.2 glass available from Corning Incorporated as Code 7059. This glass is nominally free of alkali metal oxides. This means that the glass has no intentionally added alkali metal compound in its batch, but may contain up to about 0.1% Na.sup.+ as an impurity. Use of this glass avoids contamination of the liquid crystal.
In the active device, the back panel, or active plane, has thin film transistors formed on the glass by photolithographic steps, together with attached circuitry. The front panel, or color plane, has transparent colored dots or stripes in the case of a full color display. Combined with the inverted design of thin film transistors, Code 7059 glass also provides sufficient protection for the active elements in an active matrix LCD device while the amorphous silicon transistors are being fabricated on the glass at temperatures under 400.degree. C. Sodium contamination would lead to transistor instability over time.
However, use of polysilicon thin film transistors is becoming increasing popular for active matrix LCDs. This practice involves processing temperatures that are substantially higher than the temperatures required for the amorphous silicon, and that approach the strain point of the glass. At these temperatures, sodium, which is present as an impurity, begins to create a problem again. As a result, it again becomes necessary to provide a barrier layer on the glass panel to prevent migration.
Fabrication of the active plane, or active matrix, involves the use of multiple photolithographic steps which require precise alignment. This requires that the panels not only have precise dimensions as formed, but that such precise dimensions be retained during subsequent processing steps. However, these processing steps may involve thermal exposure at or near temperatures where a glass may undergo structural rearrangement and/or dimensional relaxation. Accordingly, it has become common practice to subject glass panels to a compaction process after formation and before further thermal processing.
Compaction involves reheating a glass body to a temperature below the glass softening point, but equal to or above the maximum temperature reached in a subsequent processing step. This achieves structural rearrangement and dimensional relaxation in the glass prior to, rather than during, the subsequent processing. Preliminary compaction is imperative where it is necessary to maintain precise alignment and/or flatness in a glass body during subsequent photolithographic processing, as in the manufacture of flat panel display devices.
It is economically attractive to compact glass sheets in stacks. However, this necessitates interleaving, or separating, adjacent sheets with a release material to avoid sticking. At the same time, it is necessary to maintain the sheets extremely flat, and with an optical-quality surface finish.
The panels used in an LCD device must, of course, be of optical quality. Strict cleanliness is a requirement during all processing. Any marring of the surface, such as surface scratches, indentations, or the like, must be avoided.
Currently, sheets of graphite are inserted between glass panels to serve as a parting agent during the compaction process. They must be removed at completion of the process. This is not only an added step, but on occasion leads to scratches that must be removed by polishing.
It has been proposed in U.S. Pat. No. 5,073,181 (Foster et al.) to substitute a monolayer of submicron silica particles as a parting layer. However, this is also a non-permanent layer that must be removed before further processing. Hence, it could not function as a barrier layer to sodium migration.
It would, therefore, be desirable to apply a permanent surface film on at least one side of a glass panel during formation of the panel, or prior to further processing. This film would have to remain on the panel and not interfere with processing or operation of an LCD display, either passive or active. The film should be inert, transparent and refractory. It should serve as a parting agent to prevent glass adhesion during compaction, and also as a barrier layer to prevent sodium migration which would result in liquid crystal, or thin film transistor, degradation. Finally, it should improve the scratch resistance of the surface. It is a basic purpose of the present invention to provide a method of producing a glass panel for an LCD device that has such features.